1. Field of the Invention
This invention relates to a current transformer and current transformer system which are used to measure an alternating current passing in, for example, a main circuit of electric power distribution equipment or substation main circuits equipment, and especially relates to a current transformer and digitalized electronic current transformer system using a Rogowski coil.
2. Description of the Related Art
Generally, a penetrated type current transformer is used in many cases for measuring alternating current passing in electric power distribution equipment, substation main circuit equipment and so on. In a conventional penetrated type current transformer, a secondary winding is wound around a toroidal former, that is a core, and a conductor in which a primary current passes penetrates a centered opening of the core. An iron core, or non-ferromagnetic material is used as the core of this penetrated type current transformer. Among these, a current transformer using a non-ferromagnetic material is called a air core coil type current transformer or a Rogowski coil, which can acquire excellent linearity characteristics without saturation.
FIG. 14 shows the structure of a common Rogowski coil. The Rogowski coil 1 shown in this figure is constituted by a conductor winding 2 coiled from point P to point Q on all over the periphery of the core 6 made of the non-ferromagnetic material, and returning a wire (return circuit line) 3 from point Q to point R in a direction opposite to a winding direction of the winding 2 along the core 6. The return circuit line 3 usually returns between the core 6 and the winding 2. Moreover, a conductor 5 of a main circuit of electric power distribution equipment or substation equipment penetrates an opening 6a of the core 6.
In this situation, voltage is generated proportional to amount of time differential of the primary current flowing in the conductor 5 between terminals 4, 4 of the winding 2 and the return circuit line 3. Accordingly, the above-mentioned primary current can be measured by integrating this voltage and multiplying a constant determined by a form of the coil. For an ideal Rogowski coil, the voltage between terminals 4, 4 is not influenced by a gap of centered points of the core 6 and the conductor 5, and by magnetic field of outside the Rogowski coil 1. An ideal Rogowski coil satisfies the following conditions: (a) a winding interval (pitch) of the winding 2 is constant, (b) an area surrounded by the winding 2 is equal to an area surrounded by the return circuit line 3, (c) cross-sectional area of the core 6 is fixed over the entire circumference and not influenced by temperature, and (d) the winding 2 is completely wound over the entire circumference of the core 6 without any missing portion.
However, when manufacturing the Rogowski coil 1 as shown in FIG. 11, it is technically difficult to satisfy the above-mentioned condition (a), that is, to wind the winding 2 to the core 6 while keeping a constant winding interval. Although a fixed winding interval can be maintained by preparing slots or projections to the core 6 for fixing position of the winding 2, a special core and winding machine is necessary for this preparation, and thus increases the price of the Rogowski coil which becomes very expensive.
In order to solve this disadvantage, a conventional structure of the Rogowski coil shown in FIG. 15 has been constituted. In the Rogowski coil 1 shown in this figure, a metal foil 2e is formed on both sides of a printed circuit board 7 having an opening 9 penetrated by a conductor 5 at a central part so as to coincide with straight lines radially spreading from the center of the opening 9. Moreover, the winding 2 and the return circuit line 3 is constituted so that the radially-arranged metal foils 2e of one side surface of the printed circuit board 7 and the metal foils of a reverse side surface thereof are electrically connected by plated holes which penetrate the printed circuit board 7.
In this example shown in FIG. 15, the return circuit line 3 is formed in the shape of winding, thus, the output voltage between the terminals 4, 4 per unit current and unit frequency becomes large, and sensitivity of the Rogowski coil 1 improves. In addition, winding progress direction of the winding 2 is in a clockwise rotation, and that of the return circuit line 3 is in a counterclockwise rotation. This construction is shown such as a Japanese Patent Disclosure (koukai) No. 6-176947, which is a counterpart of the U.S. Pat. No. 5,414,400. According to such conventional technology, by applying general technique of manufacturing printed circuit boards, the Rogowski coil 1 can be inexpensively manufactured while keeping winding intervals of the winding 2 and the return circuit line 3 constant. Therefore, it becomes possible to realize the condition (a) mentioned above to a remarkable degree. Hereinafter, a Rogowski coil whose windings are constituted by metal foils arranged on the printed circuit board is called as a printed circuit board type Rogowski coil.
By the way, in the conventional Rogowski coil mentioned above, the condition (b), that is, the condition of making an area which the winding 2 surrounds and an area which the return circuit line 3 surrounds equal, cannot be fulfilled completely. This makes it easier for the Rogowski coil to be influenced by an external magnetic field, and this gives rise to an error at the time of current measurement increases.
FIG. 16 is a pattern diagram showing a situation that magnetic flux Φ, due to an external magnetic field in a direction of penetrating an opening 6a at the center of the core 6, interlinks the winding 2 of the common Rogowski coil 1 as shown in FIG. 14. FIG. 17 is a pattern diagram showing a situation that the same magnetic flux Φ due to the external magnetic field interlinks the return circuit line 3 of the common Rogowski coil 1 as shown in FIG. 14.
Since the winding progress direction of the winding 2 is reverse of that of the return circuit line 3, the voltage generated between the terminals 4, 4 of the Rogowski coil 1 shown in FIG. 14 is equal to a difference of the voltage generated between the points P and Q shown in FIG. 16 and the voltage generated between the points P and Q shown in FIG. 14. Assuming that the magnetic flux Φ due to the external magnetic field is uniform all over the surface of the Rogowski coil 1, if the area A, designated by diagonal hatched lines in FIG. 16, which the winding surrounds is not equal to the area B, designated by diagonal hatched lines in FIG. 17, which the return circuit line surrounds, a voltage due to the exterior magnetic field is generated between the terminals 4, 4. Since this voltage is unrelated to the primary current which should originally be measured, it causes a measurement error.
Factors that give rise an external magnetic field are explained below. For example, the external magnetic field is generated when a bend exists in the conductor 5 or when a current flowing conductor 8 exists near the Rogowski coil 1, as shown in FIG. 18, or when the conductor 5 is arranged at an angle to the Rogowski coil 1 as shown in FIG. 19. When applying the Rogowski coil 1 to an actual electric power distribution main circuit equipment or substation main circuit equipment, it is impossible to completely eliminate the above-mentioned factors. In addition, since usually an actual magnetic flux Φ due to the external magnetic field is not uniform, the influence becomes still more complicated.
It is possible to reduce an error by completely making the area A which the winding surrounds and the area B which the return circuit line 3 surrounds equal, more preferably, by arranging the form of the winding 2 and the form of the return circuit line 3 to be completely identical. However, in the common Rogowski coil 1 shown in FIG. 14, it is difficult to manufacture while controlling the area the return circuit line 3 surrounds being constant, thus it is very difficult to avoid the influence of an external magnetic field. On the other hand, though the Rogowski coil shown in FIG. 15 reduces the influence of an external magnetic field considerably, there is still the influence of the external magnetic field because of the constitutional reason that the area the return circuit line 3 is smaller than the area the winding 2 surrounds.
Now, although the influence of the external magnetic field to the Rogowski coil has been explained so far, another problem is explained here. That is, although it has been stated that the influence of the external magnetic field can be considerably reduced by adopting the Rogowski coil as shown in FIG. 15, there is still a problem that the common Rogowski coil shown in FIG. 14 cannot be simply replaced with the Rogowski coil shown in FIG. 15. The reason of the impossibility of the simple replacement of the coils is that, a scale of the secondary output voltage of the Rogowski coil shown in FIG. 15 in terms of the primary current (the scale corresponds to a current transformation ratio in case of an iron core type current transformer) cannot be raised to a level of that of the common Rogowski coil shown in FIG. 14.
As known well, the secondary output voltage of a Rogowski coil is proportional to the product of a number of turns of the coil and a cross-sectional area of one turn coil. As for the common Rogowski coil shown in FIG. 14, the secondary output voltage in terms of the primary rated current is usually several tens of volts per kilo ampere. In the Rogowski coil shown in FIG. 14, since cross section of one turn coil can be decided arbitrarily as long as restrictions of an attachment space allow, and since the number of turns of the coil can be adjusted so that required secondary output voltage may be obtained, by means such as double winding or triple winding, several tens of volts per kilo ampere can be obtained easily as the secondary output voltage. If the several tens of volts per kilo ampere can be obtained as the secondary output voltage from the Rogowski coil, an analog voltage signal can be transmitted without being influenced by noises from the power distribution main circuit equipment or substation main circuit equipment in the field where the Rogowski coil is installed to the main control building of electric power installation where a protection units and control units are affected, that is, without degradation of a signal which effects the protection units and the control units.
However, for the Rogowski coil shown in FIG. 15, there is a physical limit in a number of turns of a coil and a size of cross section of one turn coil in the winding, because of a structural reason that the coil winding is composed of the metal foils formed on the printed circuit board. Although depended on the size of the printed circuit board and the width of the metal foil, a number of turns of a coil is limited to at most one thousand, and the cross section of one turn coil in the winding is restricted due to the fact that the manufacturing limit of thickness of the printed circuit board is at most 5 to 6 millimeters. Thus, the secondary output voltage of the Rogowski coil shown in FIG. 12 is limited at most 100 mV/kA. Though it is assumed that ten sheets of the Rogowski coils are connected in series, the secondary output voltage is about 1 V/kA, and, from a viewpoint on withstanding transmission noise, it is difficult to transmit an accurate analog-voltage signal to a main control building of an electric power installation.